Cementation of natural hydrocarbon production wells is performed between the external part of the casing and the wall of the well to support the entire series of tubes and to prevent the flow of pressurized liquid or fluid originating in the subterranean strata in which drilling occurs.
It is especially important that the cement used for this purpose exhibits, after hardening, rapid and pronounced resistance to compression in order to possess satisfactory mechanical properties. Furthermore, it is often necessary that these cements are gas-impermeable in order to effectively block the flow of highly-pressurized gases which may be present in the drilled strata and which may travel through the cement, even as the cement hardens.
However, frequently, the drilled strata exhibit a mechanical strength which is too weak to allow the use of cements having a high density, i.e., of approximately 1.6 to 1.9 g/cm.sup.3. In this case, recourse must be made to cements termed "lightened," i.e., whose density ranges between 1 and 1.6 g/cm.sup.3 and in which inert or active lightening filling materials are incorporated.
The state of the art has proposed numerous types of filling materials. For example, the use of hollow glass microspheres containing air or an inert gas has been suggested. However, these filling materials pose serious risks for pumps and injection systems, since they may burst or implode. Furthermore, the real density of the cement grout is difficult to adjust. Finally, these lightening products are costly.
Silicates have also been proposed as lightening filling materials, but these compounds have a low compression resistance.
Finally, incorporating compounds capable of producing a gas in situ, which forms bubbles in these compounds, into the cement grout has also been contemplated. This solution can plausibly be considered for areas where the depth is slight. However, this procedure is more costly and is difficult to implement because the diameter of the ga bubbles cannot be controlled effectively and the bubbles promote the cracking of the cement.
In any case, the gas-impermeability of the cements lightened by these means poses serious problems for which solutions have long been sought.
Patent No. FR-A-2587988 proposes the use of, in a hydraulic cement slag containing light aggregates as a lightening filling material, such as hollow microspheres, an impermeability agent formed by particles of silica powder accounting for between 5 and 100% by weight of the hydraulic cement. These silica particles may be secondary products originating in electric ovens used for the production of silicon or ferrosilicon.